A highly stable oscillator is an oscillator which can perform stable oscillation by keeping the temperature of an atomic resonator for determining an oscillation frequency to a constant value. For example, OCXO (Oven Controlled Xtal Oscillator: temperature-controlled type crystal oscillator) and a rubidium atomic oscillator are known as such a highly stable oscillator.
FIGS. 1A and 1B are diagrams showing the construction of a general highly stable oscillator, wherein FIG. 1A is a diagram showing the construction of OCXO, and FIG. 1B is a diagram showing the construction of a rubidium atomic oscillator.
As shown in FIG. 1A, OCXO contains a thermostatic chamber 2 in which a crystal oscillator 1 serving as a atomic resonator is mounted, and an oscillation circuit network 3, and the temperature of the crystal oscillator 1 is kept constant with respect to the external ambient temperature in the thermostatic chamber 2. In OCXO, an oscillation signal based on the natural vibration frequency of the crystal oscillator 1 is input to the oscillation circuit network 3, and an oscillation output is output from the oscillation circuit network 3.
In such OCXO, a heater is disposed in the thermostatic chamber 2, and a control device for keeping the temperature of the heater constant is disposed in the thermostatic chamber 2 or the oscillation circuit network 3, for example. The control device will be described later with reference to FIG. 2.
As shown in FIG. 1B, the rubidium atomic oscillator contains thermostatic chambers 2A, 2B and 2C. A rubidium lamp cell 4 serving as an atomic resonator is mounted in the thermostatic chamber 2A, a filter cell 5 is mounted in the thermostatic chamber 2B, and a resonance gas cell 6 and a cavity resonator 7 are mounted in the thermostatic chamber 2C. Accordingly, each of the rubidium lamp cell 4, the filter cell 5, the resonance gas cell 6 and the cavity resonator 7 is kept to a constant temperature. Gas cell transmission light output from the resonance gas cell 6 is subjected to photoelectrical conversion in a photocell 8, passed through a preamplifier 9 and then input to the oscillation circuit network 3. An oscillation output is output from the oscillation circuit network 3.
In the rubidium atomic oscillator as described above, heaters are disposed in the thermostatic chambers 2A to 2C, and a control device for keeping the temperature of the heater constant is disposed in each of the thermostatic chambers 2A to 2C or the oscillation circuit network 3, for example. The control device will be described later with reference to FIG. 2.
As described above, according to the highly stable oscillator, the atomic resonator for determining the oscillation frequency is kept to a stable and constant temperature with respect to the external ambient temperature, thereby stabilizing the oscillation frequency of the oscillation output.
Here, in general, the temperature characteristic of the crystal oscillator of OCXO is determined by cut angle, and thus the set temperature (target value) of the thermostatic chamber 2 is frequently set to 70° C. or more in accordance with the ambient temperature of equipment to which the highly stable oscillator is applied. It is a rare case that the set temperature of the thermostatic chamber 2 of OCXO exceeds 100° C. at most. The set temperatures of the thermostatic chambers 2A to 2C of the rubidium atomic oscillator are generally set to 70° C. to 120° C.
As described above, the set temperature of the thermostatic chamber of the highly stable oscillator is generally set to 70° C. or more, and the upper limit temperature based on the environmental condition of the applied equipment is generally equal to 70° C. or less. Therefore, a heat type temperature control circuit is used as the temperature control circuit of the highly stable oscillator.
In the case of a highly stable oscillator in which the set temperature of the thermostatic chamber is relatively low (for example, 25° C.), a Peltier device which can increase or reduce the temperature is frequently used.
However, the heat efficiency for reduction of temperature is lower than that for increase of temperature, and thus there is a drawback that the highly stable oscillator is designed in a large scale. Since OCXO and the atomic oscillator are generally restricted in use space, a cooling type thermostatic chamber is not used, but a temperature increasing type thermostatic chamber is used.
The circumstances surrounding highly stable oscillators such as OCXO and the rubidium atomic oscillator have been recently severe, and further miniaturization has been required. Furthermore, high speed and high capacity designs have been promoted in equipment such as a computer and a server to which highly stable oscillators are applied, and thus the internal temperature of the equipment is increased. Therefore, the highly stable oscillators have been required to withstand a higher ambient temperature.
Here, in order to withstand a higher ambient temperature, the setting temperature of the thermostatic chamber is required to be increased. This is because a highly stable oscillator must be designed in a large scale when a cooling type thermostatic chamber is used, and thus a temperature increasing type thermostatic chamber is enabled to be used by setting the set temperature to a value higher than the ambient temperature, thereby avoiding the large-scale design of the highly stable oscillator.
The change of the temperature characteristic of the atomic resonator which is required in such a case as described above is implemented by changing the cut angle of the crystal oscillator in the case of OCXO and by changing the kind, pressure or the like of filler gas in the rubidium lamp, the filter cell and the gas cell in the case of the rubidium atomic oscillator.
FIG. 2 is a diagram showing the construction of a thermostatic-chamber temperature control device for performing temperature control of a thermostatic chamber of a general highly stable oscillator.
A conventional thermostatic-chamber temperature control device contains a heater 10 for increasing the temperature of the thermostatic chamber, a bridge circuit 11 having a temperature sensitive element 11A, an operational amplifier 12 for outputting the voltage corresponding to an unbalanced voltage of the bridge circuit 11, and a transistor 13 having a base to which the output of the operational amplifier 12 is output.
The heater 10 is a heater wire for keeping the temperature of the thermostatic chamber constant, and it is secured to the housing of the thermostatic chamber. The heater 10 is connected between a power supply (Vcc) and the collector of the transistor 13.
The temperature sensitive element 11A is an element such as a thermistor whose resistance value varies in accordance with the surrounding temperature, and it is provided to detect the temperature of the heater 10. The temperature sensitive element 11A is installed in the bridge circuit 11 disposed between the power supply (Vcc) and the operational amplifier 12.
The bridge circuit 11 is disposed between the power supply (Vcc) and the operational amplifier 12. The bridge circuit 11 contains the temperature sensitive element 11A and three fixed resistors, and is disposed so that the unbalanced voltage thereof is input to the input terminal of the operational amplifier. Here, when the resistance value of the temperature sensitive element 11A is represented by Rt(Ω) and the resistance values of the three fixed resistors are represented by Ra(Ω), Rb(Ω) and Rc(Ω) respectively, the balance condition of the bridge circuit 11 is represented by Rt=Ra×Rc/Rb. The resistance value Rt of the temperature sensitive element 11A is set so as to be equal to Ra×Rc/Rb(Ω) when the temperature of the thermostatic chamber is equal to the set temperature as the target value.
The base of the transistor 13 is connected to the output terminal of the operational amplifier 12, the collector of the transistor 13 is connected to the power supply (Vcc) through the heater 10, and the emitter of the transistor 13 is connected to the ground.
The operational amplifier 12 contains resistors R1 and R2 and a capacitor (C0) for voltage division, and a resistor R3 is connected between the operational amplifier 12 and the transistor 13.
In the thermostatic-chamber temperature control device as described above, when the temperature of the thermostatic chamber decreases in connection with decrease of the ambient temperature and thus an unbalanced voltage is output, the transistor 13 is driven in accordance with the output of the operational amplifier 12. When the transistor 13 is turned on and thus collector current flows, the heater 10 is turned on, and the temperature of the thermostatic chamber is increased. When the temperature of the thermostatic chamber reaches a target value, the unbalanced voltage is equal to zero, and thus the transistor 13 is turned off. As described above, the temperature of the thermostatic chamber is kept constant.
Next, the relationship of the ambient temperature of the thermostatic chamber and each of the current flowing in the heater 10 (heater current), the power consumption of the heater 10 and the power consumption of the transistor 13 will be described.
FIG. 3 is a characteristic diagram showing the relationship of the heater current and the power consumption with respect to the ambient temperature of the thermostatic chamber. In the characteristic diagram shown in FIG. 3, the abscissa axis represents the ambient temperature, the ordinate axis at the left side represents the power consumption of the heater 10, and the ordinate axis at the right side represents the current value of the heater 10.
When the ambient temperature of the thermostatic chamber increases, the unbalanced voltage of the bridge circuit 11 decreases, so that the transistor 13 is driven to reduce the heater current. On the other hand, when the ambient temperature decreases, the unbalanced voltage of the bridge circuit 11 increases, so that the transistor 13 is driven to increase the heater current. Here, the power consumed in the heater 10 is obtained by multiplying the square of the current flowing in the heater 10 by the resistance value of the heater 10.
The power consumption of the transistor 13 is shown in FIG. 4.
FIG. 4 is a characteristic diagram showing the power consumption of the transistor with respect to the ambient temperature. In FIG. 4, the abscissa axis represents the ambient temperature, and the ordinate axis represents the power consumption of the transistor 13. It is assumed that the set temperature (target value) of the thermostatic chamber is set to 80° C.
When the power supply voltage is represented by Vcc(V), the heater current is represented by I(A), the resistance value of the heater 10 is represented by RH(Ω), the voltage between the collector and the emitter is represented by VCE(V) and the voltage drop in the heater 10 is represented by RH×I(V), the power supply voltage Vcc is constant, and thus VCE is represented by the following formula (2).VCE=Vcc−RH×I  (2)Therefore, the power consumption Ptr(W) in the transistor 13 is determined according to the following formula (3).Ptr=(Vcc−RH×I)×I  (3)Accordingly, it is apparent that the power consumption in the transistor 13 is maximum when the current I is equal to a half of the maximum current value as shown in FIG. 4.
Here, when the ambient temperature is equal to 0° C., the resistance value between the collector and the emitter of the transistor 13 is substantially equal to zero. Therefore, the power consumption of the transistor 13 is equal to zero as shown in FIG. 4. At this time (when the ambient temperature is equal to 0° C.), the power consumption of the heater 10 is maximum (Vcc2/RH).
Furthermore, when the ambient temperature is equal to the set temperature, that is, 80° C., the transistor 13 is turned off, and no current flows in the heater 10, so that the power consumption of the transistor 13 is equal to zero.
Accordingly, in the case where the set temperature of the thermostatic chamber is set to 80° C., it is found that the power consumption of the transistor 13 is maximum when the ambient temperature is about 40° C.
The maximum power consumption of the transistor 13 is equal to about a quarter of the maximum consumption power of the heater 10.
Next, reliability of the transistor 13 will be considered. In general, an upper limit under a use environment which is represented as a junction temperature (for example, 150° C.) is set to the chip temperature of a transistor.
When the ambient temperature of the transistor 13 is represented by TA(° C.), the thermal resistance is represented by θjA(° C./W) and the power consumption of the transistor is represented by Ptr(W), the temperature Ttr(° C.) of the transistor 13 in the thermostatic-chamber temperature control device shown in FIG. 2 is represented by the following formula (4).Ttr=TA+θjA×Ptr  (4)
The thermal resistance θjA is a value which also varies in accordance with a transistor chip being used, the pattern of a print board on which the transistor chip is mounted or radiation heat from a portion at which the transistor chip is disposed, and the value obtained by multiplying the thermal resistance θjA by the power consumption of the transistor 13 shown in FIG. 4 approximately corresponds to the temperature increase of the transistor chip. In light of the characteristics shown in FIG. 4, the power consumption of the transistor 13 is maximum around the half temperature point of the set temperature of the thermostatic chamber. Therefore, the chip temperature of the transistor 13 is estimated to be maximum at the center temperature of the use environment. This is also satisfied in a case where a FET (Field Effect Transistor) is used in place of the transistor 13.
The highly stable oscillators have been recently required to be further miniaturized. Therefore, it is impossible to physically separate the thermostatic chamber from a switching element such as the transistor or FET for driving the heater.
When the thermostatic chamber and the switching element are in close proximity to each other as described above, the switching element suffers thermal conduction from the thermostatic chamber, and heating of the switching element itself is also added, so that the chip temperature of the switching element may be equal to the temperature of the thermostatic chamber or more.
The increase of the chip temperature of the switching element promotes deterioration of the switching element formed of a semiconductor, and the lifetime of the switching element is shortened, which may cause problems such as long-term reliability degradation.
Particularly, miniaturization of equipment and diversification of the types of applied equipment have been increasingly required for recent highly stable oscillators, and thus a thermostatic-chamber temperature control device having higher reliability has been required to be developed.